Plastic materials are generally used for building water heaters of the type mentioned above, in order to avoid the drawbacks of traditional apparatus comprised of metal tanks which need to be specially arranged to withstand corrosion.
Document E2-A-0 674 139 shows a water heater made by joining two shells of a single plastic material, in particular polypropylene.
Document FR-A-2 620 803 shows a water heater with a shell of thermoinsulating expanded material, coated with a polyurethane foam jacket.
All water heaters of a known type mentioned above have a small capacity and cannot stand high pressures, for example operating pressures of 8 bar, at temperatures of up to 90.degree.-100.degree.. They can stand, however, peak pressures of at least 16 bars as required by the field regulations in force. In addition, water heaters of a known type need to be made with rather thick layers of insulating expanded material in order to avoid a rapid loss of heat towards the outside.